1. Field of the Invention
The present invention is directed to the field of electrochemical gas analyzers. More particularly, the present invention is directed to that portion of the above-noted field which is concerned with the generation of an electrical signal indicative of a gas chemistry. More specifically still, the present invention is directed to that portion of the above-noted field which is concerned with electrochemical gas sensors responsive to the partial pressure of oxygen in gaseous samples. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with the generation of an electrical signal indicative of the partial pressure of oxygen within the heated gaseous combustion by-products generated by an internal combustion engine. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with the generation of an electrical signal which may be rendered relatively insensitive to changes in the temperature of the gaseous combustion by-products while responding rapidly to variations in the partial pressure of oxygen in the gaseous combustion by-products.
2. Description of the Prior Art
It has been determined that the operation of a conventional automotive internal combustion engine produces gaseous combustion by-products including hydrocarbons, carbon monoxide and various oxides of nitrogen. Various efforts are being made to reduce the quantity of such by-products. Extensive investigation into the combustion process, examination of alternative combustion processes and detailed studies of exhaust gas treatment devices have lead to the conclusion that the use of a catalytic converter within the exhaust system of an internal combustion engine provides a practical and effective technique for substantially reducing the emission of the gaseous combustion by-products into the atmosphere. A catalytic exhaust treatment device or converter which is capable of substantially simultaneously converting all three of the aforementioned combustion by-products into water, carbon dioxide, and gaseous nitrogen is referred to as a "three-way catalyst." However, for the known three-way catalyst devices to be most effective, the gaseous by-products introduced into the converter must be the by-products of combustion of a substantially stoichiometric air/fuel mixture. Such three-way catalysts are said to have a very narrow "window" of air/fuel ratios at which the device is most efficiently operative on these three combustion by-products. By way of example if .lambda. is the air/fuel ratio normalized to stoichiometry, the window may extend from about 0.99.lambda. to about 1.01.lambda.. Such a three-way catalyst converter is described, for example, in U.S. Pat. No. 3,895,093 issued to Weidenback et al. on July 15, 1975, assigned to KaliChemi Aktiengesellschaft and titled Catalytic Removal of Carbon Monoxide Unburned Hydrocarbons and Nitrogen Oxides From Automotive Exhaust Gas. For air/fuel ratios of the combustion mixture on either side of the window, one or two of the aforementioned combustion by-products will be converted in only very small percentages. Within the window, the three by-products will be converted at very high percent efficiencies approaching 90% in some cases. In view of the narrowness of the catalytic converter window, it has been determined that the associated internal combustion engine should be operated with a combustible mixture having an air/fuel ratio as close as possible to stoichiometry.
The most satisfactory technique for assuring continuous or substantially continuous operation at the desired air/fuel ratio is through the utilization of an appropriate feedback control mechanism. In implementing suitable feedback control systems, it has been proposed to employ sensors responsive to the chemistry of the exhaust gases, that is, the hot gaseous by-products of combustion, in order to control the precise air content and/or fuel content of the air/fuel mixture being provided to the engine.
One type of electrochemical exhaust gas sensor employs a ceramic material which demonstrates a predictable electrical resistance change when the partial pressure of the oxygen of its environment changes. An example of such a material is titania (titanium dioxide having a general formula TiO.sub.2). Such sensors can be fabricated generally in accordance with the teachings of U.S. Pat. No. 3,886,785 issued to Stadler et al., titled Gas Sensor and Method of Manufacture and assigned to the assignee hereof. Tests of such devices have shown that at elevated and substantially constant temperatures, the devices will demonstrate a virtual step change in resistance for rich-to-lean and lean-to-rich excursions of the air/fuel ratio of the combustion mixture producing the exhaust gas environment of the device.
A principal difficulty which has been encountered with such variable resistive devices resides in the fact that such devices will demonstrate a measurable resistance change which is also a function of change of the temperature of the ceramic material, for example a change of about 500.degree. F. produces a measurable resistance change on the order of magnitude of a sensed rich-to-lean or lean-to-rich air/fuel mixture change. Such a temperature variation can be encountered, depending of course to some extent on the location of placement of the sensor within an exhaust system, during acceleration of the associated engine from idle speed to highway speeds. Heretofore, exhaust gas sensors which employed a variable resistance sensor ceramic have required that the temperature of the material be relatively closely controlled for reliable use in a feedback system intended to provide an internal combustion engine with very precise air/fuel ratio control.
Temperature control of the associated sensor has required the addition of expensive electronic temperature sensing and heating control systems external to the exhaust conduit and the addition of a heater element per se situated internally of, or in close proximity to, the sensor element. In order to narrow the operational range of temperature of the sensor, the sensor has been operated at the higher end of the predictable range of exhaust gas temperatures thus requiring substantially continuous application of heat energy for most of the operating cycles of the associated engine. While such devices have continued to be of rugged construction, the addition of the heater and associated electronics devoted to temperature control have increased cost and have increased statistical failure problems. An additional problem which has been encountered is a ceramic fracture problem believed to be associated with thermal shock caused by the rapid heating of the ceramic material by the heater element. For less precise operation, such devices have been required to be installed at a location in an exhaust gas environment where the temperature of the exhaust gases will not vary substantially for variation in the operating cycle of the associated engine.
Since variable resistance exhaust gas sensor devices are of substantially greater mechanical strength and ruggedness than are other known types of exhaust gas sensor and are not subject to the temperature gradient which is inherent in operation of a galvanic cell type of exhaust gas sensor, it is an object of the present invention to provide a variable resistance exhaust gas sensor construction which is relatively temperature insensitive. With greater particularity, it is an object of the present invention to provide a titania exhaust gas sensor construction which is capable of producing an output signal which is rendered relatively insensitive to the temperature of the surrounding environment. With greater particularity still, it is a further and particular object of the present invention to provide a variable resistance ceramic exhaust gas sensor construction which is relatively insensitive to the temperature of the surrounding medium and which need not be maintained at a substantially constant temperature. With the foregoing objective in mind, it is a further object of the present invention to provide an exhaust gas sensor which does not require the application of external heating energy. It is also a further and particular objective of the present invention to provide a means of temperature compensation for a variable resistance ceramic exhaust gas sensor whereby sensor performance over a relatively wide range of operating temperatures will be relatively temperature insensitive. In furtherance of the foregoing objectives, it is a further and particular objective of the present invention to provide a variable resistance ceramic exhaust gas sensor with temperature compensation in the form of a high temperature thermistor in a construction which is rugged in use and which does not require expensive manufacturing techniques or equipment.
One attempt known in the prior art for satisfying the above requirements is to provide a pair of electrically series connected variable resistance ceramic sensor elements. One of the ceramic sensor elements is a variably resistive partial pressure of oxygen responsive and temperature responsive ceramic such as, for example, titania. The other of the ceramic sensor members is a variably resistive temperature responsive thermistor. The ceramic sensor members are connected electrically in series and are arranged to define a voltage divider network. When a reference voltage is applied across the voltage divider network, the voltage appearing at the junction between the ceramic sensor members may define the output voltage of the exhaust gas sensor construction. The voltage appearing at the junction of the sensor elements and the voltage divider network will be relatively temperature independent since temperature effects on the ceramic members will be electrically complementary. By comparing the output voltage to the voltage level at either end of the voltage divider network a useful output signal may be derived. By selectively referencing to define the output signal as either the voltage drop across the partial pressure of oxygen responsive member or the voltage drop across the thermistor sensor member, the resulting output signal can be rendered to be responsive to the air/fuel ratio. The use of two such elements in the exhaust gas sensor construction increases the statistical chances of failure. Further, it is desirable to obstruct the flow of exhaust gases as little as possible and the use of two such sensors obstructs it more than the use of a signal sensor.